1. Field of the Invention
The present invention relates to a lens system and, for example, a lens system preferably used as an image-taking optical system of an image-taking apparatus such as a digital video camera, a digital still camera and the like.
2. Description of the Related Art
In recent years, in addition to cameras using silver salt films, for example, 35 mm-size cameras, digital video cameras and digital still cameras using solid-state image pickup elements such as CCD sensors and CMOS sensors have been rapidly popularized as new categories of image-taking apparatuses.
In such digital still cameras and digital video cameras, there has been a strong demand for higher fineness in taken images and downsizing of the image-taking apparatus bodies, and accordingly, combining a high resolution power and downsizing has been required in the image-taking systems (image-taking lenses). In particular, for low-profile cameras which attach importance to portability, image-taking lenses with a short entire lens length have been strongly requested. For a shorter whole lens length, it is advantageous to reduce the number of component lenses as small as possible.
In addition, in terms of a camera using a solid-state image pickup element, if the distance from the exit pupil of the image-taking lens to the image plane is extremely short, shading occurs since an incident angle of an off-axis light ray into the light receiving plane is great. Accordingly, for such an image-taking lens to form an image on a solid-state image pickup element, a telecentric optical system where the exit pupil is sufficiently distant from the image plane is requested.
As an embodiment to realize a telecentric optical system, a so-called retro focus type is known wherein a lens unit on an object side beyond an aperture stop has a negative refractive power and a lens unit on an image side beyond the aperture stop has a positive refractive power. In a retro focus-type image-taking lens, a front lens component and a rear lens component which are positioned across an aperture stop both act so as to make an angle formed by the off-axis principal ray and optical axis small, this is preferable to maintain telecentricity.
As a retro focus-type image-taking lens, for example, a retro focus-type image-taking lens with a 3 units and 4 elements structure of a negative lens, a stop, a cemented lens, and a positive lens is known (Patent document 1, for example).
In addition, a retro focus-type image-taking lens with a 3 units and 4 elements structure of a negative lens, a stop, a positive lens, and a cemented lens is known (Patent documents 2–14, for example).
In addition, a retro focus-type image-taking lens with a 4 units and 4 elements structure of a negative lens, a stop, a positive lens, a negative lens, and a positive lens is known (Patent document 15, for example).
In addition, a retro focus-type image-taking lens with a 4 units and 5 elements structure of a negative lens, a stop, a positive lens, a cemented lens having a positive refractive power, and a negative lens is known (Patent document 16, for example).
In addition, a retro focus-type image-taking lens with a 4 units and 5 elements structure of a negative lens, a stop, a positive lens, a cemented lens having a positive refractive power, and a positive lens is known (Patent document 17, for example).
In addition, a retro focus-type image-taking lens with a 5 units and 5 elements structure of a negative lens, a stop, a positive lens, a negative lens, a positive lens, and a positive lens is known (Patent documents 18 and 19, for example).
In addition, a retro focus-type image-taking lens with a 4 units and 4 elements structure of a negative lens, a positive lens, a stop, a negative lens, and a positive lens is known (Patent document 20, for example).    [Patent document 1] U.S. Pat. No. 5,251,073    [Patent document 2] U.S. Pat. No. 5,724,190    [Patent document 3] Japanese Patent Application Laid-Open No. H10(1998)-048515
(Corresponding U.S. Pat. No. 5,796,528)    [Patent document 4] Japanese Patent Application Laid-Open No. H09(1997)-179022    [Patent document 5] Japanese Patent Application Laid-Open No. H09(1997)-033802    [Patent document 6] Japanese Patent Application Laid-Open No. H08(1996)-005908    [Patent document 7] Japanese Patent Application Laid-Open No. H11(1999)-119093    [Patent document 8] Japanese Patent Application Laid-Open No. H10(1998)-142496    [Patent document 9] Japanese Patent Application Laid-Open No. H05(1993)-288985    [Patent document 10] Japanese Patent Application Laid-Open No. H09(1997)-090218    [Patent document 11] Japanese Patent Application Laid-Open No. H10(1998)-300906    [Patent document 12] Japanese Patent Application Laid-Open No. H10(1998)-301025    [Patent document 13] Japanese Patent Application Laid-Open No. H11(1999)-038316    [Patent document 14] Japanese Patent Application Laid-Open No. H11(1999)-109233 (corresponding to U.S. Pat. No. 6,118,592)    [Patent document 15] Japanese Patent Application Laid-Open No. H10(1998)-048514    [Patent document 16] U.S. Pat. No. 5,781,350    [Patent document 17] Japanese Patent Application Laid-Open No. 2000-002835    [Patent document 18] Japanese Patent Application Laid-Open No. 2001-100094    [Patent document 19] Japanese Patent Application Laid-Open No. 2002-098887    [Patent document 20] Japanese Patent Publication No. H05 (1993)-037288
In a retro focus-type image-taking lens, in order to provide a compact lens structure with narrowed lens intervals, it is preferable to strengthen a negative refractive power of the front lens component and to compose the rearmost lens unit (a lens closest to the image side) of a positive lens. At this time, it is necessary, in order to maintain telecentricity, to strengthen refractive power of the rearmost lens. However, if the refractive power of the rearmost lens of the rear lens component becomes excessively strong, barrel-shaped distortion and astigmatism greatly occur. For avoidance thereof, the front lens component and rear lens component must be arranged apart to some extent, and consequently, it is very difficult to realize a lens system to satisfy requests such as downsizing, telecentricity, and good optical performance.
In order to maintain telecentricity while realizing a compact lens structure, it is necessary that an angle formed by an off-axis principal ray and an on-axis principal ray which are directed from the rearmost lens toward the maximum image height is in a certain range. When these are taken into consideration, none of the image-taking lenses as disclosed in the above-described patent documents 1–20 were sufficient for a lens system to form an image in a solid-state image pickup element.